An image display device having been invented so far to provide an observer with an image displayed on an LCD or similar display element as a virtual image is exemplified by the invention disclosed in the Patent Literature 1. In the image display device of Patent Literature 1, the light emitted from an LED array is diffused by a diffusing plate and is led into the LCD. The image light from the LCD is then led to an optical pupil through optical elements such as a prism or lens. When the pupil of the observer is set to the position of the optical pupil, the observer is allowed to see the image displayed on the LCD.
Incidentally, to facilitate observation of an image (virtual image) by the observer, it is essential to brighten the image while expanding the optical pupil. In this case, if the incident light is diffused in all directions by a diffusing plate, the brightness of the light will be reduced, even if the optical pupil can be expanded in all directions. Thus, it is very effective to use a diffusing plate that diffuses incident light in one direction (unidirectional diffusing plate), instead of a diffusing plate that diffuses the incident light in all directions. To be more specific, the unidirectional diffusing plate is used to diffuse the incident light in one direction and to form a big optical pupil in one direction, whereby easier observation of the image by an observer is ensured. Further, the light from the light source is converged in the direction perpendicular to the aforementioned direction of diffusion so that the observer is provided with a brighter image.